Structure of the atom

Dr Jeremy Jones et al.

The structure of the atom is key to the majority of the techniques used in radiology, and a general understanding of atomic structure is worthwhile.

The following is an uncomplicated overview of the structure of the atom.


The composition of an atom is principally 'empty space'. Its mass is concentrated within the central nucleus which is composed of a specific number of nucleons. Nucleons are either protons or neutrons, and the total number of nucleons is assigned the symbol A (also known as the mass number). The total number of protons within the nucleus is called the atomic number and assigned the symbol Z. Protons and neutrons have a mass of 1. Protons are positively charged, and neutrons have no charge.

  • A (mass number): total number of nucleons = protons + neutrons
  • Z (atomic number): number of protons

A great many different configurations of the nucleus may occur. In each case, the atomic number defines the element, i.e. it is the number of protons that determines that carbon (having 6 protons) is carbon. However, more than one nuclide may exist for a given element, i.e. carbon may exist with 6 neutrons (carbon-12) or 8 (carbon-14): in each case, there are still 6 protons.

Orbiting electrons

Orbiting the positively charged nucleus is a cloud of electrons, which have negligible mass and a negative charge. For an element to have a neutral overall charge, the number of electrons will be equal to the number of protons in the nucleus.

The electrons orbit the nucleus in a similar way to planets orbiting the sun (Rutherford–Bohr model). They orbit in one of the 'shells' that surround the nucleus: these are named K, L, M, N etc. from the centre outwards. Only a certain number of electrons may occupy a particular shell:

  • K: 2 electrons
  • L: 8 electrons
  • M: 8 electrons

In each atom, the outermost shell is called the valence shell and may only be partially full. It has a role to play in the properties that the atom has.


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Article Information

rID: 5304
Section: Physics
Synonyms or Alternate Spellings:

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